Liquid flow meters are well known, and one such type of flow meter is described in U.S. Pat. No. 3,867,840, BAATZ, in which the liquid to be measured is supplied to a chamber in tangential direction. A rotor is retained within the chamber, the liquid being removed from the chamber in axial direction. The rotor carries vanes. As the fluid is introduced into the chamber, termed a swirl chamber, it impinges on the rotor vanes to rotate the rotor. Rotation is sensed by an optical-electrical evaluation device, the speed of the rotor, and the number of revolutions per unit time, determining the flow speed of the liquid, and hence the quantity of liquid being supplied.
The rotor has some inertia. Liquid which is in the chamber, and even after new liquid has been supplied, will continue to swirl therein, thus continuing further rotation of the rotor although no further liquid is being supplied. The structure necessarily requires some clearance between the vanes on the rotor and the walls of the swirl chamber, which permits some liquid to leak past the rotor. The accuracy of measurement at extremely low fluid flow, thus, is impaired by the possibility of such leakage from the inlet to the outlet duct in the swirl chamber.
It has also been proposed to locate a rotor directly within a fluid duct--see German Patent Disclosure Document DE-OS No. 29 11 826, WERKAMM. The rotor is essentially drum-shaped and is formed with helical ribs at its circumference. Axial ribs are located within the duct upstream from the rotor in order to direct the fluid flow in axial direction with respect to the rotor and to render it uniform. Fluid passing the rotor will meet the helical ribs, thus rotating the rotor.
The rotor, of course, must have some clearance within the duct in which it is retained and some of the fluid will flow along the duct without contacting the ribs of the rotor. For extremely low flow quantities, the measuring results, therefore, will not be accurate, and the fluid meter will not only be non-linear but may give random outputs not in accordance with actual flow. Further, if flow is stopped, the rotor will continue to run on for some time, so that the run-out of the rotor also will cause measurement errors.